basement/garage under slab insulation?
buildzilla
| Posted in Energy Efficiency and Durability on
working a build with basement and garage in cz-6.
i see a lot of references to under-slab insulation,
and it’s obvious that applies to slab-on-grade builds,
but not obvious if those same principles apply to basement or garage slabs.
in my case there would be a bonus-room over the garage, so i have some increased motivation to keep the garage area as warm as possible,
i might also end up with some portion of the basement finished,
so basically my question is:
should i insulate under these slabs and if so, with what, and what should i be thinking about in terms of rough roi.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
As for the basement, I would certainly insulate under the slab (EPS) and on the walls (EPS or polyiso), following installation guidelines easily available by search on this site. The garage is a different matter. Certainly the above grade walls ought to be insulated, and the ceiling ought to be detailed with an air barrier between garage and room above and insulated well. Insulating under the garage slab really depends on whether or not you will be conditioning that space regularly. In either case, I advise insulating the foundation walls down to footings. If heating the garage then insulate the slab. If not heating it, then skip the insulation under the slab, so that ground heat will moderate the interior temperature.
I had a house built back in 2010-11 in CZ6 (central NH), superinsulated and with attached garage, not superinsulated, but with 2x6 framing and FG batts and cellulose blown overhead. The garage is not heated, but its foundation walls are insulated to R10. The slab is not insulated. From what I have seen, the floor never goes below freezing, typically bottoming out in upper 30s. Any snow or ice that melts off one of the cars just puddles on the floor and stays liquid. Last weekend, when it got to -15 F at bedtime one night, the next morning I looked at a thermometer on a shelf a few inches above the slab and well back from the door. It read +38 F.
i don't think i would be conditioning the garage unless i happen to be working out there in the cold for a bit, so i'll take your advice under consideration. maybe i'll insulate it if i get a deal on reclaimed eps, or have some leftovers. 38F last weekend in your garage is amazing!
Yes, the principals apply to basement slabs.
I assume you're doing a basement garage, with a negative sloped driveway? vs a basement under the garage, with a 2nd floor bonus room on top of that?
Either way, I think its valuable to insulate under the sub grade slab, just the same as an at-grade slab. However, pay attention to the insulation at the ceiling of the garage (the floor of the bonus room) as well of the walls. Garage doors are notorious for air leaks and aren't well insulated. Consider a higher heating load for that bonus room. I'm sure the HVAC experts on here can talk more about strategies on the bonus room.
Other things to consider are air sealing the walls/ceiling of the garage to the house and including ventilation for better IAQ of the house.
There are other posts on here regarding ROI of insulation. Look into BeOpt.
Jamie
Given the high cost for sub slab insulation if you are not actively heating the garage it seems unlikely it would ever save enough energy to recover the cost.
There are so many factors in ROI that the only real answer would be a computer model of your home with you entering the cost per sqf from your bids and your guesses at the future cost of fuel. About 20 hours of training and work.
https://www.nrel.gov/buildings/beopt.html
https://www.youtube.com/playlist?list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d
Walta
thanks walta, now what's the roi on my time attempting to figure out the roi, lol
If the foundation walls are insulated down to at least the frost line all the way around the garage, there is less benefit to insulating the slab. If this won't be conditioned space, I wouldn't bother insulating the slab since you won't really be accomplishing much -- if you put no energy in to begin with (in terms of heating), there is no energy to "save" by using extra insulation.
Regarding your bonus room, I'd insulate the floor and then consider a layer of polyiso under the joists before the ceiling drywall goes up in the garage.
Bill
that makes sense, i'll at least insulate the frost walls and i guess slab perimeter.
i'll def do what i can for the floor of the bonus-room above, my son's bedroom is above the garage of my current house and it's noticeably colder than the rest of the house.